Field of the Invention
Embodiments of the present invention relate to a current resonant type switching power supply device such that oscillation of a feedback control loop is prevented, thus obtaining stabilization of a switching operation.
Discussion of the Background
FIG. 5 is a schematic configuration diagram of a current resonant type switching power supply device typified by an LLC type of converter device. This kind of current resonant type switching power supply device includes first and second switching elements Q1 and Q2, connected in series to form a half-bridge circuit, that alternately switch an input voltage Vin. The first switching element Q1 is connected in series to a primary coil P of an insulating transformer T in series via a capacitor Cr and inductor Lr. Also, the second switching element Q2 is connected in parallel to the primary coil P of the insulating transformer T via the capacitor Cr and inductor Lr. Further, an output circuit formed of diodes D1 and D2 and an output capacitor Cout is connected to secondary coils S1 and S2 of the insulating transformer T.
The switching power supply device is such that, after the first switching element Q1 is turned on, causing current to flow through the primary coil P, the second switching element Q2 is turned on, generating current resonance in a resonant circuit formed of the capacitor Cr, inductor Lr, and a leakage inductor Lm of the insulating transformer T. The switching power supply device is such that alternating current is caused to flow through the primary coil P of the insulating transformer T by the heretofore described operation being repeated. Also, the output circuit is such that alternating voltage generated in the secondary coils S1 and S2 of the insulating transformer T is rectified via the diodes D1 and D2 and smoothed in the output capacitor Cout, whereby a predetermined output voltage Vout is obtained.
CONT in FIG. 5 is a control circuit that alternately drives the first and second switching elements Q1 and Q2. The control circuit is formed of an oscillator that generates a drive signal of a switching frequency fs. Also, VS is an output voltage detector circuit that detects the output voltage Vout and generates a feedback signal FB in accordance with the output voltage Vout. The control circuit CONT controls the switching frequency fs of the first and second switching elements Q1 and Q2 in accordance with the feedback signal FB provided from the output voltage detector circuit VS, thereby keeping the output voltage Vout constant.
FIG. 6 schematically shows the concept of a feedback control loop of the switching power supply device. The control circuit CONT alternately drives the first and second switching elements Q1 and Q2 in a switching power supply device main body SW at the switching frequency fs. Also, the output voltage detector circuit VS detects the output voltage Vout of the switching power supply device main body SW, and generates the feedback signal FB in accordance with the detected output voltage Vout. The feedback signal FB is formed of a feedback voltage VFB or a feedback current IFB. The control circuit CONT controls the switching frequency fs (=f(Vout)) by converting the frequency of this kind of feedback signal FB. For example, as introduced in detail in JP-A-2009-171837 (“PTL 1”), the input/output voltage ratio (voltage conversion ratio) in the switching power supply device main body SW is changed, and the output voltage Vout is kept constant, by control of the switching frequency fs.
Herein, the input/output voltage ratio in the switching power supply device changes depending on the switching frequency fs, and changes as shown in FIG. 7 in accordance with the size of a load resistance Ro. Specifically, the input/output voltage ratio increases along with an increase in the switching frequency fs and, after reaching a maximum at a certain frequency fr, gradually decreases, as shown in FIG. 7. Further, when the switching frequency fs reaches a resonance frequency fo of the current resonant circuit, the input/output voltage ratio becomes “1”.
Therefore, the switching power supply device is normally configured so that the input/output voltage ratio is constantly in a step-up mode of “1” or more. Specifically, the switching power supply device is configured so as to carry out modulation control of the switching frequency fs in accordance with the feedback signal FB between the frequency fr, at which the input/output voltage ratio reaches the maximum value, and the resonance frequency fo.
However, the previously described control of the switching frequency fs, that is, frequency modulation control, is realized by variably controlling the switching frequency fs linearly, or variably controlling a switching cycle Ts linearly, in accordance with the feedback voltage VFB or feedback current IFB provided as the feedback signal FB. The switching cycle Ts has a relationship of (fs=1/Ts) with respect to the switching frequency fs. Consequently, when the switching cycle Ts is linearly controlled, there are non-linear control characteristics such that the lower the feedback voltage VFB, the greater the change in the switching frequency fs, as shown by a solid line in FIG. 8.
Herein, when linear control of the switching frequency fs is executed when the input voltage Vin or output voltage Vout of the switching power supply device changes, gain in the resonant circuit rises considerably along with an increase in the feedback voltage VFB, as shown by a broken line in FIG. 9. Further, when the feedback voltage VFB increases, gain in the previously described feedback control loop increases. Thereupon, the feedback control loop itself oscillates, and there is concern that operation will become unstable.
Regarding this point, by executing linear control of the switching cycle Ts, the rise in the resonant circuit gain can be suppressed more in comparison with when executing linear control of the switching frequency fs, as shown by a solid line in FIG. 9. Conversely, however, when the feedback voltage VFB decreases, the switching frequency fs increases in comparison with when executing linear control of the switching frequency fs, and the feedback control loop gain also increases. Thereupon, for example, there occur problems such as inrush current, or the like, being liable to occur when the switching power supply device is started-up.